At the URL https://help.libreoffice.org/latest/en-GB/text/swriter/01/04120212.html (please have a look at end of the page)
we can see that is possible to sort an alphabetic index by numeric key value but only “alphanumeric”
field is available. Any advice ?
The help page states:
Key type
Select numeric when you want to sort numbers by value, such as in 1, 2, 12. Select alphanumeric, when you want to sort the numbers by character code, such as in 1, 12, 2.
(bold attribute added)
So, we must look for another way “to sort an alphabetic index by numeric key value”. I don’t know how.
The Writer Guide 7.6 has this on page 351:
Sort
Defines how the entries are sorted when displayed. The only option is alphanumeric, but you can define which language alphabet will be used.
See also: Bug 119345 - Writer: Key type “numeric” for sorting in an index is missing